1. Field of the Invention
The present invention relates to an antenna, and more particularly, to an antenna for use in a portable telephone, such as a cellular phone.
2. Description of the Related Arts
In light of the developments of wireless communications technology and the reduction of terminal prices, portable phones such as cellular phones and personal communications service (PCS) phones have been popularized and are widely being used. In a portable phone, an antenna assembly typically includes a whip antenna having an electrical length of .lambda./4 and a helical antenna which is attached on the top end of the whip antenna and has an electrical length of .lambda./4 also.
One example of conventional antenna assemblies having such a configuration is illustrated in FIG. 1. In the antenna assembly of FIG. 1, a helical antenna 12 is installed displaced by a certain space from the top end of an antenna rod of the whip antenna 10, so that undesired resonance due to an electromagnetic coupling between the whip antenna 10 and the helical antenna 12 is negligible. When a portable phone employing the antenna assembly shown in FIG. 1, is in a stand-by state, a user of the phone usually retracts the antenna into the phone. In this position, only the helical antenna is effective on the antenna characteristics. Meanwhile, when the portable phone is in a call-attempt or a call receiving state, the user usually extends the antenna from a housing of the phone. In such a case, the helical antenna as well as the whip antenna affects the antenna characteristics along.
FIG. 2 shows another example of conventional antenna assemblies. In FIG. 2, a helical antenna 22 is attached to the top end of an antenna rod of a whip antenna 20. In the conventional antenna assembly of FIG. 2, the helical antenna 22 is in constant electrical connection with the antenna rod. When the antenna assembly of FIG. 2 is in its extended position, the helical antenna 22 of .lambda./4-length and the whip antenna 20 of .lambda./4-length show a combined characteristics of an antenna of .lambda./2-length. Meanwhile, when the antenna assembly of FIG. 2 is in its retracted position, only the helical antenna 22 of .lambda./4-length affects the antenna characteristics.
The antenna assembly of FIG. 1 or FIG. 2, is disadvantaged in that the bandwidth is narrow. Thus, the radiation efficiency thereof is not good, particularly when the antenna assembly is in its retracted position. Furthermore, the portable phone employing the antenna assembly has to be provided with ample space therein for receiving the antenna assembly when the antenna assembly is retracted. This creates an obstacle in the reduction of the size of the phone.
FIG. 3 shows yet another example of conventional antenna assemblies. A helical antenna 32 is attached and fixed on a housing of the phone and a whip antenna 30 is installed so as to be movable upward and downward through the inside the helical antenna 32. In such an antenna assembly, the whip antenna 30 and the helical antenna 32 are operative in parallel when the whip antenna 30 is extended, while only the helical antenna 32 is operative when the whip antenna 30 is retracted. While the antenna assembly of FIG. 3 has the advantage of having the helical antenna 32 fixed firmly on the phone, the antenna assembly also has the disadvantage of requiring more space inside the phone. The antenna assembly of FIG. 3 requires more space inside the phone because the top end of the whip antenna should be lower than the bottom end of the helical antenna in order to avoid an undesired electromagnetic coupling between the helical antenna and the whip antenna when the whip antenna is retracted.
Meanwhile, the size of the portable phones is generally getting smaller to enhance the convenience in carrying the phone. Also, due to the transition of the frequency bands employed in wireless communication systems towards higher frequencies, the components in the phone and the phone itself are further being miniaturized in their size. In light of the trend of miniaturization, it is desired to reduce the physical length of the portion of the antenna assembly received in the housing body of the portable phone when the antenna assembly is in a retracted position, along with the receiving space in the housing body. Meanwhile, it is also desired that the bandwidth of the helical antenna is enlarged and the antenna characteristics are stabilized. Additionally, it is desired to achieve these objectives while minimizing the size of the antenna.